1. Field of the Invention
The present invention relates to an apparatus and method for the feedback control of respiratory gas flow within a mechanical breathing assist apparatus and in particular to an apparatus and method for the adaptive feedback control of the gas flow.
2. Description of the Prior Art
Feedback controllers are used within a mechanical breathing assist apparatus, such as a ventilator system, to adjust gas flow rates based on a measurement of a system gas parameter, for example gas pressure, rise time, or flow rate, in order to achieve and maintain the value of that variable at or within an operating range of a target value. These controllers are usually operably connected to a flow control regulator, such as a solenoid valve, to provide a control signal used to adjust the opening of the valve. How the adjustment is made to reach the target value depends not only on the measured value of the system flow parameter that is fed back to the controller, but also on additional parameters known as control parameters. These control parameters directly affect the performance and stability of the controller and their optimal values may change with time as system properties, such as compliance and resistance, vary. A mechanical breathing assist apparatus is particularly problematical to control in this manner since its pneumatic system includes (or is connected in use to) a patient""s respiratory system, including lungs, the compliance and resistance of which can change unpredictably with time and with patient.
In order to overcome this problem it is known to provide controllers having control parameters which automatically vary or xe2x80x9cadaptxe2x80x9d with changes in properties of the ventilator system. One such controller which provides an adaptation for a next breath that is based on the analysis of gas delivery in previous breaths is disclosed in U.S. Pat. No. 5,271,389. This controller has a comparator for periodically generating in a current breathing cycle, an error signal representing the difference between a value of a gas parameter measured for gas within the system and a target value of the gas parameter, a control signal generator for processing the error signal in accordance with a control function having a variable value control parameter to generate a control signal usable in the regulation of the respiratory gas, and adaption means for varying the value of the variable value control parameter responsive to the error signal. The adaption means operates by summing the error signal in a particular period with all past error signals for the corresponding period of past breaths to provide a cumulative error signal which is used to vary the control parameter for the same period of the next breath. Thus the correction will xe2x80x9cimprovexe2x80x9d as the error values from more breaths are added to the cumulative signal.
It is an object of the present invention to provide an adaptive feedback control of gas flow within a mechanical breathing aid system in which the adaption is made on a breath-by-breath basis without the need to rely on a cumulative error signal.
The above object is achieved in accordance with the principles of the present invention in a feedback controller for regulating respiratory gas flow in a mechanical breathing assist apparatus, having a comparator which periodically generates, in a current breathing cycle, an error signal which represents a difference between a value of a gas parameter that was measured for gas within the system, and a target value for this gas parameter, a control signal generator which processes the error signal using a control function having a control parameter with a variable value, to generate a control signal used to regulate the respiratory gas, and an adaptation unit for varying the value of the control parameter dependent on the error signal, by determining, for the current breathing cycle, an extreme value of the periodically generated error signal and by varying the value of the control parameter for a next breathing cycle dependent on the rate of change of the extreme value of the error signal relative to the value of the control parameter in the current breathing cycle.
By providing for the adaptive variation of a variable control parameter for a subsequent breathing cycle which is based on the change of an extreme error signal value (that is a maximum or a minimum value depending on how the error signal is derived and which phase of the breathing cycle is being controlled) with the value of the control parameter, then disturbances in the breathing assist apparatus system are automatically compensated, based on the past performance of the system and typically based on the performance of the system over consecutive breathing cycles, without the need to establish a cumulative error signal.
Preferably an integral gain control parameter is varied assuming a linear relationship between an extreme pressure error signal and the value of the integral gain parameter. In a feedback controller, such as a PID or PI controller in which respectively a xe2x80x9cproportional-integral-derivativexe2x80x9d or a xe2x80x9cproportional-irtegralxe2x80x9d control function is implemented, the integral gain parameter that is found to be highly sensitive to disturbances in the pneumatic system such as changes in lung resistance and compliance.